Kim Jong Il, the North Korean dictator who ruled over one of the poorest, most belligerent and isolated countries in the world, has died, state television reported Monday.

Kim died of heart failure while travelling by train on Saturday at 8:30 a.m. local time, the country's state-run news agency reported. State media said the 69-year-old died of a heart ailment on a train because of a "great mental and physical strain" during a "high-intensity field inspection."

NKTV endorsed his son and heir apparent Kim Jong Un in a statement released Monday. But his death fueled speculation about a struggle for power in the reclusive nation, and with that control of a nuclear arsenal and vast military.

"Our people and the military pledge to uphold the leadership of comrade Kim Jong-Un," NKTV said in the statement.

Having been in power since taking over from his father Kim Il-sung in 1994, Kim announced in September 2010 that his third son, Kim Jong Un, would be his successor, and the young man, believed to be in his late 20s, was given several high-ranking posts.

After taking power, Kim isolated his country from the outside world to protect his dictatorship and made North Korea into a nuclear power, which drew global condemnation.

Kim's pursuit of nuclear weapons led to sanctions that helped keep most of his people in poverty, but despite his austere image as supreme ruler, he was rumored to be a playboy who famously wore platform shoes and had an affinity for women, wine and American movies.

He seemed to swing erratically between willingness to discuss his country's nuclear program with the United States, China, Japan, Russia and South Korea, and rejecting any outside pressures.

In 2009, North Korea pulled out of talks aimed at ending its nuclear program, and kicked weapons inspectors out of the country, which led to new stricter sanctions from the United Nations.

Speculation over Kim's health has been rampant since he reportedly suffered a stroke in August 2008.

Since the reported stroke, Kim appeared strikingly old and frail, with visible hair loss. Medical experts in Seoul had said they believed the hair loss was not only from aging, but also a result of kidney dialysis and diabetes.

Yet during the early part of a visit to Russia in August, Kim looked vibrant and seemed to have recovered from his suspected stroke. He was spotted using left hand to hold a tray and even to sign the visitor's book at the hydro plant, which many analysts in Seoul said was meant to demonstrate to the world that he was fit and strong. He had not been able to move his left arm last year.

Analysts also agreed that Kim appeared to have gained weight, which suggested he had recovered.

But when he arrived at the summit with Russian President Dmitry Medvedev, he looked frail. Reports showed that the younger Medvedev had to support the 69-year-old's elbow when shaking hands, and Kim showed signs of fatigue. He was also spotted needing help using the stairs.

Given his unquestioned dictatorial power in a nation that poses a world nuclear threat, the status of his health has been a constant subject of speculation among analysts, medical experts, and intelligence agencies that scrutinize occasional photographs and rare videos of Kim.

South Korea's Yonhap News pointed out last year that Kim's hands had been darkening, while his nail color was abnormally whitening. Citing numerous medical analysts, Yonhap said that indicates that Kim appeared to be suffering from kidney malfunction which is one of the symptoms of diabetes.

Analysts began searching for evidence of the presumed stroke ever since two French doctors told French newspaper Le Figaro in December 2008 that they had treated the North Korean leader for cerebral infarction.

"If you look at the few available videos of Kim walking recently, he limps dragging his left leg. His left arm is also just hanging there without movement," said Chul-Joong Kim, medical consultant and chief medical correspondent for South Korea's major newspaper Chosun Ilbo.

Others have pointed out that when he appeared on North Korean television in July 2010, Kim's lips were twisted, suggesting facial paralysis symptoms.

His apparent weight loss had also been a subject of many guesses. Some South Korean reports cited pancreatic cancer, while others inferred that he was going through a rigorous diet program to prevent another stroke.

Much like his heir apparent son, Kim held several high posts in the government before formally leading the communist nation. After the death of his father, Kim became chairman of the North Korea's National Defense Commission, which was considered the highest declared position within the government.

Kim soon led the Communist Worker's Party, which sealed his position as leader of the country.

Kim's nuclear weapons stronghold and his strategic partnerships within the region were viewed as a threat by many democratic governments. Kim cited national security reasons for his nuclear weapons production program. In 2006, his regime successfully executed its first underground nuclear test.

Conflicts between North and South Korea continued throughout Kim's rule. Tensions seemed to have cooled in recent years as South Korea sent humanitarian aid through U.N. agencies to help alleviate the famine in North Korea. Talks between the two nations' leaders seemed to have added to the illusion.

Still, Kim moved forward with his nuclear weapons program. In 2002, President George W. Bush said in his State of the Union address that North Korea was part of the "axis of evil," along with Iraq and Iran.

Hopes for reconciliation between the two nations diminished when North Korea attacked the South Korean island of Yeonpyeong, killing two South Korean marines and two civilians.

South Korea's military has been put on alert following the death announcement.

The White House said in a short public statement that it is in close contact with South Korea, and it is "closely monitoring" reports of the death. There are currently about 29,000 U.S. troops stationed in South Korea.